1. Field of the Invention
The present invention relates to information recording media on which information is recorded, erased, rewritten, and reproduced optically and electrically, methods for producing the same, and methods for recording/reproducing information thereon.
2. Description of the Prior Art
There are phase-changeable information recording media as information recording media on which information is recorded, erased, rewritten and reproduced using a laser beam. Information is recorded, erased, and rewritten on the phase-changeable information recording media utilizing the phenomenon that a recording layer is changed reversibly between a crystalline phase and an amorphous phase. In general, when recording information, the recording layer is melted and cooled abruptly with irradiation of a laser beam so that the irradiated portion is changed to an amorphous phase. On the other hand, when erasing information, the recording layer is irradiated with a laser beam having a lower power than that for recording to warm the recording layer and cool it gradually, so that the irradiated portion is changed to a crystalline phase. Therefore, in the phase-changeable information media, it is possible to record or rewrite new information while erasing recorded information by irradiating the recording layer with a laser beam that changes its power between a high power level and a low power level (see “Basis and Application of Optical Disk Storage” by Tsunoda Yoshito et al., The Institute of Electronics, Information and Communication Engineers, 1995, Chapter 2).
In recent years, various techniques have been under research to provide a large capacity to information recording media. For example, a technique for achieving high density recording by using a blue-violet laser having a short wavelength or using a thin substrate on the side from which a laser beam is incident and a lens having a large numerical aperture NA to reduce the spot diameter of the laser beam is under research. Another technique under research is such that using information recording media provided with two information layers, the two information layers are recorded/reproduced with a laser beam incident from one of the two information layers (see JP 12-36130 A). This technique, which uses two information layers, can achieve about twice the recording density.
In the information recording medium including two information layers on/from which information is recorded/reproduced from one side (hereinafter, may be referred to as a “two-information-recording-layer” medium), a laser beam transmitted through the information layer on the laser beam incident side (hereinafter, referred to as a “first information layer”) is used to record/reproduce information on/from the information layer on the side opposite to the laser beam incident side (hereinafter, referred to as a “second information layer”). Therefore, it is preferable that the transmittance of the first information layer is at least 40%. On the other hand, it is desired that the second information layer has a high recording sensitivity (recording marks can be recorded even with a low power laser beam) with respect to the recording characteristics, and has a high reflectance with respect to the reproduction characteristics.
In order to achieve 40% of the transmittance of the laser beam through the first information layer, the first recording layer is required to have a small thickness of about 6 nm. However, if the recording layer is thin, the number of crystalline nuclei formed is small when crystallizing the recording layer. In addition, the distance over which atoms can move is short. Consequently, the crystallization rate of a thinner recording layer among recording layers made of the same material tends to be relatively lower. Therefore, as the recording layer is thinner, it is more difficult to form the crystalline phase, so that the erasure ratio is reduced.
Conventionally, as a material (phase-changeable material) of the recording layer, GeSbTe based materials having a high crystallization rate, excellent repeated rewriting performance and high reliability have been used. Using these materials, optical disks for computer data recording or optical disks for video recording have been commercialized. Among the GeSbTe based materials, quasi-binary compositions on the GeTe—Sb2Te3 line have the highest crystallization rate. The inventors of the present invention conducted recording/reproducing tests, using a red laser having a wavelength of 660 nm. The results are that in recording with a high linear velocity of 9 m/s, a satisfactory erasure ratio of 30 dB was obtained even if the recording layer made of GeTe—Sb2Te3 was as thin as 6 nm. This technique led to the feasibility of the two-information-recording-layer medium using a red laser.
Furthermore, information recording media in which a phase change is caused in the recording layer made of a phase changeable material by applying current have been under research. In these information recording media, the recording layer is interposed between two electrodes. In these information recording media, when current is allowed to flow gradually through the recording layer in an amorphous phase, the recording layer is changed to a crystalline phase at a certain threshold current, and the electrical resistance is dropped abruptly. Furthermore, the recording layer can return to the amorphous phase having a high resistance by applying a large current pulse having a short pulse width to the recording layer in the crystalline phase to melt and cool the recording layer abruptly. The difference in the electrical resistance can be detected easily by regular electrical means, and therefore such a recording layer allows a rewritable information recording medium to be obtained.
In order to provide a large capacity to information recording media, two-information-recording-layer media on which information is recorded/reproduced with a blue-violet laser having a short wavelength is desired for practical use. The spot diameter of a laser beam can be reduced by using a short wavelength laser beam or using a lens having a large numerical aperture and thus higher density recording can be achieved. For recording with a short wavelength laser beam, an information recording medium that allows a small recording mark to be formed in a satisfactory shape is required. When a blue-violet laser is used, the time during which the recording layer is irradiated with the laser beam is relatively short. Therefore, in order to form small recording marks, the recording layer is required to be formed of a material having a high crystallization rate. Furthermore, the recording layer is required to be formed of a material having a large change in the optical characteristics between the crystalline phase and the amorphous phase in order to obtain a sufficient signal amplitude even from small recording marks
In the experiments of the inventors of the present invention, when a conventional two-information-recording-layer medium using a red laser was used as an information recording medium for blue-violet laser, recording marks formed in the first information layer and the second information layer were small, and thus a sufficient signal amplitude was not obtained. Regarding the first information layer, when the thickness of the recording layer was made about 6 nm to ensure a sufficient transmittance, the erasure ratio was an insufficient value of less than 15 dB. The experiments of the inventors of the present invention confirmed that a large signal amplitude can be obtained by increasing the ratio of GeTe in a quasi-binary composition on the GeTe—Sb2Te3 line. However, as the ratio of GeTe is higher, the melting point of the material tends to be higher. Therefore, as the ratio of GeTe is higher, the laser power (recording power) required for forming an amorphous phase is larger. The output of the currently available blue-violet laser is smaller than that of a red laser. Therefore, when a composition having a large ratio of GeTe was used for the second information layer on which information is recorded/reproduced with a laser beam transmitted through the first information layer, the recording power was insufficient and thus the saturated signal amplitude was not obtained.
Thus, in the two-information-recording-layer medium for use with a blue-violet laser, it is important to ensure a high erasure ratio of the first information recording layer and a high recording sensitivity of the second information recording layer.
For practical use of the two-information-recording-layer medium for use with a blue-violet laser, the first information is required to have a high transmittance and good recording and erasing performance, and the second information recording layer is required to have a high reflectance, a high recording sensitivity and good recording and easing performance. Therefore, for the practical use of the medium, it is necessary to consider materials for the first and second recording layers and the constitution of the first and second recording layers.
For materials for the recording layers in which a phase change is caused by applying current, materials containing Te as the main component is in practical use. However, conventional materials require a long time in the order of microseconds for crystallization. An electrically phase-changeable information recording medium provided with two recording layers and having good recording and erasing performance has not been in practical use yet.